Aw, Sam. We just love this guy. We also love his trademark phrase: "I can fix that." (Much better than the Warden's scary "Excuse me?" don't you think?) Sam is a young African-American man who lives in the town of Green Lake in Miss Katherine's day. He's known as "the onion man" because of the onions he sells, and the various onion-based ointments, liniments, and medicines that he makes. We don't know much about him, but we know that he's strong, attractive, and brainy enough to recite poetry to Miss Katherine.
Sam seems to be accepted, even liked, by the townspeople. They buy his onions and value his advice regarding their various ailments. But at the same time, it's pretty clear that his behavior and his opportunities are majorly restricted by his status as an African-American in nineteenth-century America. The narrator tells us explicitly that, "Sam wasn't allowed to attend classes [in the schoolhouse] because he was a Negro, but they let him fix the building" (25.24). This uneasy balance between the townspeople's reliance on Sam and their desire to exclude and demonize him because of the color of his skin erupts tragically in murderous rage when Sam is seen kissing the schoolteacher Miss Katherine.
For more deets on racial tensions in 1880s Texas, read up.